The present invention relates generally to the field of thermal dissipation devices and, more particularly, to a method and an apparatus for improving the thermal performance of heat sinks.
Thermal dissipation devices are present in a wide variety of applications. Such devices generally employ conduction, convection, or a combination of conduction and convection to dissipate heat generated by a heat source. Conduction is the transfer of heat by the movement of heat energy from a high temperature region to a low temperature region in a body. Convection is the transfer of heat from the surface of a body by the circulation or movement of a liquid or gas over the surface.
A heat sink is one example of a thermal dissipation device. A heat sink is typically a mass of material (usually metal) that is thermally coupled to a heat source and draws heat energy away from the heat source by conduction of the energy from a high-temperature region to a low-temperature region of the metal. The heat energy can then be dissipated from a surface of the heat sink to the atmosphere by convection. A well known technique of improving the efficiency of a conductive heat sink is to provide a greater surface area on the heat sink so that more heat can dissipate from the heat sink into the atmosphere by natural (or free) convection. Increased surface area is typically provided by fins that are formed on a base portion of the heat sink. The thermal efficiency of a heat sink can be further increased by employing forced convection wherein a flow or stream of air is forced over and around the surface of the heat sink.
One example of an application in which thermal dissipation devices are used is an electric device such as a microprocessor. Heat can be dissipated from an electric device through the outer surfaces of the device into the ambient atmosphere or into a stream of air driven by a fan, for example. In some cases, a heat sink is provided that is coupled to the electric device to provide more efficient dissipation of heat from the device. Heat generated by the device is transferred through an adhesive bonding agent, for example, into the heat sink. The heat then travels through the heat sink by conduction and is finally dissipated into the atmosphere by natural or forced convection. A fan is commonly employed to provide additional air flow across the heat sink to dissipate a greater amount of heat energy.
As the number of components in electric devices increases, or as the power requirements or operating speeds of the electric devices increases, the amount of heat generated can increase to a point where conventional heat sink and air convection solutions are inadequate For example, the air flow required to dissipate the greater level of heat can become excessive, or the physical size of the heat sink required to dissipate the heat can become prohibitive for the particular application.
High efficiency heat sinks such as those that can dissipate high power (more than 50 watts, for example) may require a substantial flow of air through and around the heat sink. A substantial air flow may be required because the specific heat of air is fairly low, i.e. a given amount of heat transferred to a particular volume of air results in a relatively high temperature rise of the air, thus limiting the efficiency of an air cooled heat sink. Also, a fairly high impinging velocity to ensure turbulent air flow in the vicinity of a heat radiating surface may be required to increase efficiency.
Using a fan to increase air flow around a heat sink allows a particular heat sink to be smaller in size in order to dissipate a similar amount of heat as a larger heat sink without a fan, for example. Since the thermal efficiency of a conventional heat sink is increased by increasing the air flow over the heat sink, the use of additional fans or fans capable of providing air flow with a greater velocity are ways of providing the additional air flow. A fan, however, consumes additional electrical power, contributes additional audible noise to the system, and increases system failures because of moving parts in the fan.
The limitations of an air-cooled heat sink can be reduced by using a liquid to absorb heat. It is well known that the vapor phase transition of a liquid can be exploited to provide a heat sink of a high thermal efficiency. Most heat sinks that use a liquid, however, involve a closed system with valves or pumps and seals. For example, a heat pipe uses a liquid-vapor transition to transfer heat from one end of a structure to the other. The liquid-vapor transition is not actually used to dissipate the heat to the atmosphere in such closed systems. Typically, the heat is dissipated to the atmosphere by convection, in a similar fashion as a simple air cooled, finned heat sink. Most current vapor phase heat sinks only transport heat from one place to another and leave the task of actually dissipating the heat into the atmosphere as an additional step. Air media heat sinks can be made to be very efficient, but require larger surface area, higher air flow, or exotic materials.